Various types of automatic injection devices have been described in the literature. A majority of these automatic injection devices apply dose indicator barrels, dose indicator wheels or the like which, during dose setting, are only allowed to rotate less than one single revolution. The fact that the dose indicator barrel is only allowed to rotate less than one revolution during dose setting puts a limit to the obtainable angular resolution. This limited angular resolution also limits the accuracy of the dose setting procedure.
In prior art injection devices the dose setting scale arranged on the outer surface of the barrels or wheels contains only up to 42 scale units with an incremental of 2. Thus, the accuracy when setting a dose is limited by this rather rough incremental.
Examples of “one revolution” barrels or wheel may for example be found in U.S. Pat. No. 5,725,508, EP 0 338 806 or U.S. Pat. No. 5,104,380.
WO 02/053214 discloses an automatic injection device having a dose indicator barrel capable of rotating more than one revolution. However, the injection device according to WO 02/053214 applies a linear spring to move a piston rod in the distal direction of the injection device. Evidently, an injection device applying a linear spring has a built-in axial displacement due to compressions and extensions of the linear spring along the axial direction of the injection device. This linear movement may easily be utilized to provide axial movements of the dose indicator barrel. However, it is a disadvantage that linear springs are highly non-linear in terms of force vs. compression. In addition, a linear spring exhibits relative high mechanical looses. Thus, due to the problems relating to the non-linear properties and relatively high looses there is a need for injection devices having linear and more efficient injection assisting systems.
The above-mentioned problems may be solved by applying a torsion spring instead of the linear spring. An injection device applying a torsion spring is conceptually different from linear spring-based devices in that torsion-based systems do not have a built-in axial movement of the spring assisting the user in injecting a dose of medicament from the injection device. The advantages of torsion-based injection devices are many, the greatest of these probably being that torsion springs respond in a linear manner over a large working range.
Thus, there is a need for a torsion spring-based injection device providing an improved and more user friendly dose setting procedure. It is an object of the present invention to provide such torsion spring-based injection device having an expanded dose scale with a high resolution.